The literature on the art of NOx and slag control in industrial and utility furnaces is the Leslie Pruce article "Reducing NOx Emissions At The Burner, In The Furnace, And After Combustion" appearing on pages 33-40 of the January 1981 issue of Power. This article is a comprehensive treatise dealing with the burner and furnace configurations and fuels which are factors in NOx production and control. It will serve little purpose to review all the facets of this article. What is important lies in the reference to the tangentially-fired industrial and utility furnaces in which the primary and secondary combustion air can be controlled in its quantity, velocity, and direction.
In the tangentially-fired furnace, the so-called fireball is generated by directing the burner discharge to one side of the vertical axis of the furnace to create a swirling mass of combustion. The secondary air can be proportioned between the combustion of the fireball and the outside of the fireball, which is the annulus between the fireball and the walls of the furnace.
The general objective of NOx control is to maintain the flame temperature of the fireball within certain limits. Another way of expressing this limit is the specification that the fireball will be maintained in a fuel-rich combustion, while the combustion at the periphery of the fireball will be maintained air-rich. Thus, the overall flame temperature will be held to a level which will militate against the formation of NOx.
NOx, of course, is generated with the nitrogen of the fuel and the nitrogen of the combustion air. By proportioning the amount of air initially supporting the combustion and the air secondarily entering into the combustion, the resulting NOx of both the fuel and air can be controlled. The operator of the furnace combustion empirically tuns the combustion process by proportioning the amount of secondary air placement relative to the fireball and the annulus between the fireball and the furnace wall.
In general, less than 20% of the secondary air to the fireball will maintain substoichiometric combustion which limits the flame temperature of the fireball and provides the curtain of secondary air over the furnace walls. The curtain of secondary air militates against the formation of slag on the furnace walls. All this proportioning of the air to control both the NOx and the slag requires tools of adjustment available to the furnace operator.
Concomitant with the distribution of secondary air between the fireball combustion and the curtain in the annulus formed by the fireball and the furnace walls, is the problem of maintaining the velocity of these proportions of the secondary air as the load on the furnace changes. It is fundamental that both the quantity of fuel and the quantity of air will be changed as the demand for furnace heat changes. Although the quantities of secondary air may be decreased as load is dropped on the furnace, it may be desirable to maintain the velocity of the decreased secondary air close to that velocity required to maintain combustion in the fireball and/or curtain in the annulus formed by the fireball and furnace walls. In effect, the secondary air nozzles must be constructively changed to maintain the velocity of the secondary air desired for furnace combustion conditions.
The windboxes in the corners of the furnace have the vertically adjustable air nozzles supplied through channels formed by turning vanes which direct the air from conduits arranged along the outside of the furnace wall to the windboxes. The total amount of this air supplied the channels of the turning vanes is controlled by a series of dampers well-developed in the prior art. However, the proportioning and the velocity control of the total air in the channels of the turning vanes has not been provided by controls available during furnace operation. Adjustments of the cross-sectional area of the channel to vary the proportion and velocity has had to await furnace shutdown. An adjustable control element within each vane channel is needed to determine the distribution and velocity of the total combustion air supplied to the nozzle of the windbox in order to quickly control the amount and velocity of air directed to the combustion of the fireball, and the amount and velocity of the air directed to the curtain between the fireball and the furnace wall.